Numerical control (NC) machine tools have enhanced design and fabrication since the 1940's. The early machines were manual lathes and mills that were retrofitted with stepper motors to automate motion control to increase production speeds, accuracy, and part complexity. Modern Computer numerically controlled (CNC) machines use CAD/CAM (computer-aided design/computer-aided manufacturing) work flows which make manufacturing almost as easy as pressing “print”. Desktop milling machines for making custom engraving or custom circuit boards and/or three-dimensional (3D) printers that can produce physical 3D models are now common. The digital description-file for a part can be created using many different 2D- or 3D-design software packages and subsequently shared on the Internet in order to be downloaded and reproduced wherever a rapid prototyping machine exists. This has and will continue to revolutionize the way people design and make things.
Most current rapid-prototyping machines are fundamentally continuous or analog processes. Although many fabrication machines are digitally controlled, these machines continuously cut or add material to make parts. Rapid prototyping machines can be broken into two categories: additive and subtractive fabrication machines. Additive prototyping machines typically build models by extruding a material in a in a liquid state; and then, the material hardens or is hardened after exiting the print head. For example, FDM (fused deposition modeling) extrudes a molten plastic, which cools after leaving the extruder head; and stereolithography is a process that extrudes a bead of UV curable photopolymer resin which is cured by a UV laser after it is deposited in place. Oppositely, subtractive machining processes start with a chunk of material and remove material in a controlled way to create the desired part. CNC milling machines are a common form of subtractive machining; these machines use a high-speed spindle or router to cut wood, plastic, steel, or other machineable materials. CNC milling machines have been constructed with up to the six degrees of freedom and can be built at scales that can fit on a desktop or fill industrial shops. Although both additive and subtractive rapid prototyping processes are becoming more accessible and affordable, they are still material dependent processes, and the fabrication process is not reversible. CNC milling machines continuously subtract material, and 3D printers continuously add material; although, both of these processes use digitally controlled machines, these fabrication processes are still fundamentally continuous or analog.
Digital technology has most notably revolutionized information technology and computing, but construction and fabrication are still largely continuous (analog) processes. This results in parts for products being one-off designs that cannot easily be reused; when these parts become obsolete they most likely end up in landfills. Similarly, materials made from many different types of materials are expensive and time consuming to break down for recycling. Trash is an analog concept, whereas digital materials take full advantage of reuse and recycling. Therefore, building a material from multiple materials with reversible bonds between parts would allow a multi-material product to be completely recycling and reused.
A digital material is made up of discrete parts that are the fasteners and the indexing fixture for assembly. Rather than having many complex shapes, fixtures, and jigs, which sometimes require expert assemblers and documentation, digital materials reduce the unique part count to simplify building instructions. Alexander Graham Bell, know primarily as the inventor of the telephone, also invented the space frame in order to build large kites around the year 1900. He used a tetrahedral structure that had few unique parts and tuned for stiffness. In 1907 Bell designed a 28 m tall tower using another variation of his space frame structural system. The tower weighed only 5 tons and was erected by unskilled laborers in a mere 10 days. These space frame structures had many of the same properties of a digital material; the structures were made of discrete parts with discrete joints. The space frames built by Bell had few unique parts and simplified assembly instructions.
A digital material is made up of a discrete number of parts (components) that have a finite number of connections. These digital building blocks are referred to as voxels or 3D pixels. The components can be of any size and shape, made out of various materials, and can fit together in various ways. The components of digital materials must satisfy the conditions that each component can be decomposed into a finite number of smaller geometrical shapes, that two components can only make a small finite number of different connections (links), and that the connection between any two components is reversible.
Digital parts are error correcting and self-aligning, which allows them to be assembled into structures with higher accuracy than the placement accuracy of the assembling person or machine. For example, a Lego™ set consists of discrete parts that have a finite number of joints. The male/female pin joints on the top and bottom of the Lego™ block are discrete connections, which either make or do not make a connection to another block. By contrast, a masonry construction is a continuous (analog) material; while the masonry brick is a discrete unit, the mortar in its fluid state allow one brick to be placed on top of another in an infinite number of positions. Because the joint is not discrete, masonry construction is analog while Lego™ construction is digital.
Lego™ blocks utilize a discrete pin joint has a tapered feature which allows one block to register to another and correct for placement errors below a maximum error threshold. To illustrate error correction in Lego™ blocks, a human child may have approximately 0.2 mm of hand placement accuracy, but the self-aligning feature of the Lego™ connectors correct for placement errors can allow the child to assemble structures within tolerances of approximately five microns. This is analogous to the field of digital information technology. Claude Shannon showed that near perfect communication could be achieved over a noisy channel as long as the noise was below a certain threshold which can be calculated. Similarly, a physically digital structure can self-align in order to correct for placement errors and create near perfect structures.
Structures that are created from multiple material types allow explicit control over design and optimization parameters. Digital materials can be constructed out of rigid, flexible, transparent, opaque, conductors, insulators, semiconductors, lightweight, or heavy materials. Multiple material structures could be used to build microfluidics structures or assemblies made up of active and passive electronics, optics, and/or parts with specific mechanical properties. Digital materials allow any or all of these materials can be assembled within the same assembly. A multiple material digital assembly can be built by one multi-material digital assembler machine. Multi-material 3D printers already exist, but the parts are not reversible and the material palette is limited to some rigid photopolymers and elastomers. Objet machines deposit drops of material that are cured in place. The drops of material are discrete, but they still bond to another drop in infinite possible ways. Two of the primary functional requirements for a completely digital material are discrete joints and reversible bonds between parts.
A digital material is made up of parts with reversible connections between all of the building blocks that make up the material. The materials can be assembled by a digital assembler machine; the assembler may also be a disassembler. Such a machine will have a head for disassembly and sorting. Another possibility is to use a separate machine to take on the tasks of disassembling, sorting, and delivering parts back to the assembler machine. The reversible connections allow the exact same parts to be reused and reconfigured without waste or degrading the quality of the material. For example, physically digital conductors and insulators can make reconfigurable 3D circuits. Physically digital active electronics also opens up the possibility of having discrete transistors with reversible connections to make devices such as reconfigurable ASICs or other devices that can be reprogrammed by changing the physical configuration of the parts making up a device.
Jonathan Hiller (Cornell University) has constructed a voxel assembling machine and the resulting models were shown to be reversible. This machine assembles structures made up of many spherical voxels and deposits an adhesive to bind the spheres together. The assemblies were shown to be reversible and reused by dissolving the adhesive binder and separating the parts by material type for reuse.
A press-fit interference connection may be used rather than adhesives for connecting parts. Press-fit connectors eliminate the use of adhesive binder. However, the geometry required for press fit parts adds complexity to part handling and part fabrication. Spheres are easy to manufacture and handle at many scales from many materials, and they self align when placed on a lattice. A press-fit connection is a joint that holds together by friction or micro bonding between surfaces. Press-fit connections are also referred to as interference fit, because one part is essentially interfering with the space of another. A press fit connection can be generally analyzed using the following equations:f=kx (where x is the slot width)S=x2 (S is contact surface area)k=YS (Y is the material's Young modulus)f=Ys3 (f is the force required to pull apart two slotted press-fit parts)Force, area, friction, and surface finish as well as material and fabrication tolerances greatly affect the quality and repeatability of a press-fit connection. Force between two surfaces can greatly increase amount of surface area actually in contact.
One common press-fit part design is a slotted connection which mates with another slot to create an interference fit connection. This slot acts as a clamp that flexes when its mate part interferes with the space the other part occupies. This clamping mechanism is essentially a flexure, which can be designed and tuned to exert a specific force while also providing a snap-lock release mechanism for ease of reversibility. The flexing part can be used for an interlocking mechanisms which can give a press-fit connection more strength than the material itself In other words, when two press fit parts are put in maximum tension, the material will break before the connection separates. This type of connection uses the principal of elastic averaging, which means the connection is overconstrained by making contact at many points over a large area. Elastic averaging is a nondeterministic connection, but the load capacity and stiffness are not limited. On the other hand, a kinematic design is deterministic but the stiffness and load capacity is limited. A release mechanism added to a flexure will provide controlled reversibility. This allows one part to be disconnected from the structure without putting significant force on the rest of the assembly. The force required to disassemble should be less than the force required to assembly when a release mechanism is designed into a part.
An example of a digital material is GIK. GIK (“Great Invention Kit”) is a press-fit construction kit similar to Lego™. A benefit of GIK parts is the simplicity of the slotted press fit connection design, and the simple 2.5D shapes make GIK easy to fabricate at many scales and from many different materials. George Popescu analyzed the amount of force required to connect and disconnect GIK parts. This work showed that the force to disconnect the parts was constant after approximately 10 previous connections, meaning the connection is reliably reversible. Another interesting highlight of this work showed that the amount of force required to add or remove GIK parts simultaneously grows faster than linear with respect to the number of GIK parts. Popescu created a press-fit, GIK diode ohmic junction using copper, N-doped silicon, and lead parts. This proves that active electronics can be built with digital materials.
Many part designs have been designed and explored for use in additive assembly of functional digital materials. The basic functional requirements for the parts shapes are: parts must be two-and-a-half-dimensional (2.5D) geometry; parts must be vertically assembled; finished assemblies of parts must be able to achieve near one hundred percent density; and there must be press-fit connection between parts. 2.5D parts are desired to simplify part fabrication process. Vertical assembly simplifies the automated build process. The assembly process is similar to current 3D printing machine- a print/assembly head builds structures from the top-down. One hundred percent density is desirable to fill maximum amount of space with minimal voids in a structure; however, density can then be reduced as desired. Press-fit connections between parts are used for reversibility and to avoid using an adhesive binder. These parts eliminate the need for fasteners or assembly fixture; the parts are the fasteners and fixture for assembly.